Here's my thinking.
App Store profits is derived from two areas. The cost of running the App Store (which doesn't change, because the number of developers is assumed to be fairly constant), and the revenue from games and IAPs (which is variable in nature). We also know that about 85% of developers pay next to no money (in that their revenue has a negligible impact on App Store numbers, which is why Apple is willing to reduce their cut from 30% to 15%).
As such, reducing the App Store cut to 15% for everyone would result in gross margins decreasing by way more than 15%, because your costs don't change.
Apple's App Store revenue was estimated to be about $64 billion in 2020. This is close enough to Neil Cybart's own estimates, so we will roll with that. He also estimates that the App Store has around 40% gross margins. This is because Apple has margins of about 65% for services. This includes the money from Google (which is basically pure profit), and more lucrative ones like iCloud and AppleCare. So App Store margins would be lower to balance out.
Also,
The money that Apple makes from its App Store has become a flash point for critics of Apple that argue it has too much power.
www.cnbc.com
I am not sure how many iOS developers there are. Let's go out on a limb and assume about 10 million of them actively develop for iOS. That's $1 billion a year, basically a rounding error here.
At 30%, Apple's own cut is about $19 billion (or 20 billion with developer fees). If we reduce it to 15%, Apple's cut gets reduced to about $10 billion, a 50% reduction. Assume 40% margins, Apple's costs are 11-12 billion, while their profits are about 7-7.6 billion.
So you can see that if Apple were to reduce their App Store cut to 15%, my estimates show that the App Store would effectively be run at a loss, which in turn means that Apple would be effectively subsidising it using profits from elsewhere. To break even, we take (12/64*100%) which gives us 18.75%.
TL;DR - 20% is the break-even point for the iOS App Store to be self-sustaining.
You see what would happen if developers were able to bypass iTunes and use their own payment system. Apple's earnings from the App Store would basically evaporate, and the money from developers don't even come close to covering the $11-12 billion necessary for operating costs, unless you tell me there are more than 100 million developers on iOS. The other option is to get each developer to pay $1000 a year, but this would massively disadvantage the smaller indie developers. Larger companies like Hey can easily take the hit, which is why they are the ones who ought to be paying that 30% of earnings to Apple (which gets reduced to 15% from the second year onwards).
It's not about equality, but equity. The App Store helps put every developer on an even playing field, which you lose the moment the App Store is opened up.